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Re: gEDA-user: Spooky Action . . at a distance!

On 6/7/05, Mike Jarabek <mjarabek@xxxxxxxxx> wrote:
>     Most CMOS devices like your 'HC595 have protection diodes connecting
> the inputs to the power rails.   I noticed that some of your pins are
> directly connected to the power supply.  Current could flow through
> those pins and into the device, leaving enough margin for the guts of
> the chip to work. (Eg. the voltage inside the chip was Vcc - (somewhere
> between 0.3 and 0.7) V.)

All the pins that I tie anywhere are tied high, not low.  :)  Anyway,
I found the phenominon interesting and startling, to say the least.

>     O.T. but, as a side note if you intend to manufacture these boards
> and test the board with In Circuit Test (ICT) you should probably use a
> pullup on each of the inputs you wired directly to Vcc.  Eg. something
> like a 1K or 3.3K resistor.  If you have to wire to ground use a 100 or
> 110 Ohm resistor.  The ICT machine can overdrive these resistors to test
> all the I/O's on the chips...

I have no reason for using an ICT -- these chips won't be driven any
faster than a few tens of kilohertz (even if the CPU is running at
4MHz), and I'm pleased to say that the logic is working flawlessly
first-time.

--
Samuel A. Falvo II